OCCURRENCE AND LEVELS OF ORGANIC PRIORITY POLLUTANTS IN COMBINED SEWER OVERFLOWS IN THE CHICAGO METROPOLITAN AREA

The presence of organic priority pollutants in combined sewer overflows (CSOs) may impact the collection, storage and treatment of CSOs. To gather information for the planned CSO deep tunnels and reservoirs in the Chicago metropolitan area, the occurrence and levels of organic priority pollutants in CSOs were measured in two studies conducted in the late 1980s and middle 1990s. Two types of CSOs, i.e. stored and freshly generated, were sampled in both studies at various sampling locations. In the former study, 111 organic priority pollutants comprising 30 purgeables, 57 acid/base/neutral extractables (extractables), and 24 pesticides and PCBs (P&P) were analyzed for each CSO sample collected. In response to the Clean Air Act Amendments of 1990, some of Clean Air Act VOCs were added to the list of organic pollutants, which consisted of 55 purgeables, 74 extractables, and 31 P&P, analyzed in the later study.All three classes of pollutants were detected in the samples collected, but the number of organic pollutants detected varied from location to location. Many of the frequently detected organic pollutants were similar at these locations. The total number of organic pollutants detected in at least one sample ranged from 26 to 40 at three different locations in the former study and 8 to 50 at nine different locations in the later study. Extractables with an average of 19 detects outnumbered the other two classes of compounds in the later 1980s' period. In the middle 1990s' period, more organic pollutants were found in the stored CSOs than in the freshly generated CSOs. The average number of organic pollutants detected in at least one sample was 39, including 17 purgeables, 18 extractables and 4 P&P, in the stored CSOs at three locations, and 24, comprising 9 purgeables, 12 extractables and 3 P&P, in the freshly generated CSOs at six locations. Acetone, which was not analyzed in the 1980s' study, was the most frequently detected compound in almost all locations. Toluene and chloroform were the next predominant compounds in the stored CSOs, and tetrachloroethene (PCE) and chloroform were the next predominant compounds in the freshly generated CSOs.It appeared that organic priority pollutants in CSOs were less frequently detected in the middle 1990s' period than in the later 1980s' period. The predominant pollutants detected in both studies were similar. However, the concentrations were more variable in the later study, likely because significantly more samples were collected in the study. Purgeables were the most frequently detected pollutants in the later study period. These purgeable pollutants may be stripped out during CSO storage in the reservoirs, if the reservoirs are aerated. Therefore, this factor should be kept in mind while designing an aeration system for a reservoir storing CSOs.